Tuning the Driving Force for Charge Transfer in Perovskite-Chromophore Systems.

Understanding the interplay between the kinetics and energetics of photophysical processes in perovskite-chromophore hybrid systems is crucial for realizing their potential in optoelectronics, photocatalysis, and light-harvesting applications. By combining steady-state optical characterizations and transient absorption spectroscopy, we have investigated the mechanism of interfacial charge transfer (CT) between colloidal CsPbBr3 nanoplatelets (NPLs) and surface-anchored perylene derivatives and have explored the possibility of controlling the CT rate by tuning the driving force. The CT driving force was tuned systematically by attaching acceptors with different electron affinities and by varying the bandgap of NPLs via thickness-controlled quantum confinement. Our data show that the charge-separated state is formed by selectively exciting either the electron donors or acceptors in the same system. Upon exciting attached acceptors, hole transfer from perylene derivatives to CsPbBr3 NPLs takes place on a picosecond time scale, showing an energetic behavior in line with the Marcus normal regime. Interestingly, such energetic behavior is absent upon exciting the electron donor, suggesting that the dominant CT mechanism is energy transfer followed by ultrafast hole transfer. Our findings not only elucidate the photophysics of perovskite-molecule systems but also provide guidelines for tailoring such hybrid systems for specific applications.

Nucleation and Growth of Bipyramidal Yb:LiYF4 Nanocrystals-Growing Up in a Hot Environment.

Lanthanide-doped LiYF4 (Ln:YLF) is commonly used for a broad variety of optical applications, such as lasing, photon upconversion and optical refrigeration. When synthesized as nanocrystals (NCs), this material is also of interest for biological applications and fundamental physical studies. Until now, it was unclear how Ln:YLF NCs grow from their ionic precursors into tetragonal NCs with a well-defined, bipyramidal shape and uniform dopant distribution. Here, we study the nucleation and growth of ytterbium-doped LiYF4 (Yb:YLF), as a template for general Ln:YLF NC syntheses. We show that the formation of bipyramidal Yb:YLF NCs is a multistep process starting with the formation of amorphous Yb:YLF spheres. Over time, these spheres grow via Ostwald ripening and crystallize, resulting in bipyramidal Yb:YLF NCs. We further show that prolonged heating of the NCs results in the degradation of the NCs, observed by the presence of large LiF cubes and small, irregular Yb:YLF NCs. Due to the similarity in chemical nature of all lanthanide ions our work sheds light on the formation stages of Ln:YLF NCs in general.

Understanding and Preventing Photoluminescence Quenching to Achieve Unity Photoluminescence Quantum Yield in Yb:YLF Nanocrystals.

Ytterbium-doped LiYF4 (Yb:YLF) is a commonly used material for laser applications, as a photon upconversion medium, and for optical refrigeration. As nanocrystals (NCs), the material is also of interest for biological and physical applications. Unfortunately, as with most phosphors, with the reduction in size comes a large reduction of the photoluminescence quantum yield (PLQY), which is typically associated with an increase in surface-related PL quenching. Here, we report the synthesis of bipyramidal Yb:YLF NCs with a short axis of ∼60 nm. We systematically study and remove all sources of PL quenching in these NCs. By chemically removing all traces of water from the reaction mixture, we obtain NCs that exhibit a near-unity PLQY for an Yb3+ concentration below 20%. At higher Yb3+ concentrations, efficient concentration quenching occurs. The surface PL quenching is mitigated by growing an undoped YLF shell around the NC core, resulting in near-unity PLQY values even for fully Yb3+-based LiYbF4 cores. This unambiguously shows that the only remaining quenching sites in core-only Yb:YLF NCs reside on the surface and that concentration quenching is due to energy transfer to the surface. Monte Carlo simulations can reproduce the concentration dependence of the PLQY. Surprisingly, Förster resonance energy transfer does not give satisfactory agreement with the experimental data, whereas nearest-neighbor energy transfer does. This work demonstrates that Yb3+-based nanophosphors can be synthesized with a quality close to that of bulk single crystals. The high Yb3+ concentration in the LiYbF4/LiYF4 core/shell nanocrystals increases the weak Yb3+ absorption, making these materials highly promising for fundamental studies and increasing their effectiveness in bioapplications and optical refrigeration.

Long-Range Charge Transport via Redox Ligands in Quantum Dot Assemblies.

We present a strategy to actively engineer long-range charge transport in colloidal quantum dot assemblies by using ligand functionalities that introduce electronic states and provide a path for carrier transfer. This is a shift away from the use of inactive spacers to modulate charge transport through the lowering of the tunneling barrier for interparticle carrier transfer. This is accomplished with the use of electronically coupled redox ligands by which a self-exchange chain reaction takes place and long-range charge transport is enabled across the film. We identified the different modes of charge transport in these quantum dot/redox ligand assemblies, their energetic position and kinetics, and explain how to rationally manipulate them through modulation of the Fermi level and redox ligand coverage.

Limits of Defect Tolerance in Perovskite Nanocrystals: Effect of Local Electrostatic Potential on Trap States.

One of the most promising properties of lead halide perovskite nanocrystals (NCs) is their defect tolerance. It is often argued that, due to the electronic structure of the conduction and valence bands, undercoordinated ions can only form localized levels inside or close to the band edges (i.e., shallow traps). However, multiple studies have shown that dangling bonds on surface Br- can still create deep trap states. Here, we argue that the traditional picture of defect tolerance is incomplete and that deep Br- traps can be explained by considering the local environment of the trap states. Using density functional theory calculations, we show that surface Br- sites experience a destabilizing local electrostatic potential that pushes their dangling orbitals into the bandgap. These deep trap states can be electrostatically passivated through the addition of ions that stabilize the dangling orbitals via ionic interactions without covalently binding to the NC surface. These results shed light on the formation of deep traps in perovskite NCs and provide strategies to remove them from the bandgap.

Electrochemical p-Doping of CsPbBr3 Perovskite Nanocrystals.

Lead halide perovskite nanocrystals have drawn attention as active light-absorbing or -emitting materials for opto-electronic applications due to their facile synthesis, intrinsic defect tolerance, and color-pure emission ranging over the entire visible spectrum. To optimize their application in, e.g., solar cells and light-emitting diodes, it is desirable to gain control over electronic doping of these materials. However, predominantly due to the intrinsic instability of perovskites, successful electronic doping has remained elusive. Using spectro-electrochemistry and electrochemical transistor measurements, we demonstrate here that CsPbBr3 nanocrystals can be successfully and reversibly p-doped via electrochemical hole injection. From an applied potential of ∼0.9 V vs NHE, the emission quenches, the band edge absorbance bleaches, and the electronic conductivity quickly increases, demonstrating the successful injection of holes into the valence band of the CsPbBr3 nanocrystals.

Developing Lattice Matched ZnMgSe Shells on InZnP Quantum Dots for Phosphor Applications.

Indium phosphide quantum dots (QDs) have drawn attention as alternatives to cadmium- and lead-based QDs that are currently used as phosphors in lamps and displays. The main drawbacks of InP QDs are, in general, a lower photoluminescence quantum yield (PLQY), a decreased color purity, and poor chemical stability. In this research, we attempted to increase the PLQY and stability of indium phosphide QDs by developing lattice matched InP/MgSe core-shell nanoheterostructures. The choice of MgSe comes from the fact that, in theory, it has a near-perfect lattice match with InP, provided MgSe is grown in the zinc blende crystal structure, which can be achieved by alloying with zinc. To retain lattice matching, we used Zn in both the core and shell and we fabricated InZnP/Zn x Mg1-x Se core/shell QDs. To identify the most suitable conditions for the shell growth, we first developed a synthesis route to Zn x Mg1-x Se nanocrystals (NCs) wherein Mg is effectively incorporated. Our optimized procedure was employed for the successful growth of Zn x Mg1-x Se shells around In(Zn)P QDs. The corresponding core/shell systems exhibit PLQYs higher than those of the starting In(Zn)P QDs and, more importantly, a higher color purity upon increasing the Mg content. The results are discussed in the context of a reduced density of interface states upon using better lattice matched Zn x Mg1-x Se shells.